What LED is?

LED stands for Light Emitting Diode. LED's are now found on almost every electronic device including MP3 players, DVD players, TV's, clock radios and computers. LED's have been popular for decades, however today they are cheaper, brighter and come in colors than ever before. While the incandescent light bulb has been the light of choice for at least 100 years, many believe the LED will soon replace it.

How does LED work?

LEDs (light-emitting diode) are based on the semiconductor diode. When the diode is forward biased (switched on),electrons are able to recombine withholes and energy is released in the form of light. This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor. The LED is usually small in area (less than 1 mm2) with integrated optical components to shape its radiation pattern and assist in reflection.

LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors.

LED components

Common components of an LED include: A whisker which is connected to the anode, the anvil, which is connected to the cathode, a lens to illuminate the light created for distances and a high impact plastic casing to protect the LED.

Most materials used for LED production have very high refractive indices. This means that much light will be reflected back in to the material at the material/air surface interface. Therefore Light extraction in LEDs is an important aspect of LED production, subject to much research and development.

The Color of an LED Light

LEDs are available in a variety of colors. As opposed to a white LED, that have a very small color temperature range, RGB LEDs have a limitless variety of colors that can be varied by the use of controllers

WHITE LIGHT
There are two ways of producing high intensity white-light using LEDs. One is to use individual LEDs that emit three primary colors – red, green, and blue, and then mix all the colors to produce white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb works.

RGB LED
There are several types of multi-colored white LEDs: Dichromatic, Trichromatic, and tetrachromatic white LEDs. Several key factors that play among these different approaches include color stability, color rendering capability, and luminous efficacy.
Often higher efficiency will mean lower color rendering, presenting a trade off between the luminous efficiency and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy(120 lm/W), but the lowest color rendering capability. Conversely, although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficiency. Trichromatic white LEDs are in between, having both good luminous efficacy(>70 lm/W) and fair color rendering capability. Blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) andInGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light, though white

Phosphor based LEDs

Phosphor based LEDs have a lower efficiency than normal LEDs due to the heat loss from the Stokes shift and also other phosphor-related degradation issues. However, the phosphor method is still the most popular technique for manufacturing high intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex , and the majority of high intensity white LEDs presently on the market are manufactured using phosphor light conversion.

Technically the phosphor based white LEDs encapsulate InGaN blue LEDs inside of a phosphor coated epoxy. A common yellow phosphor material is, cerium - doped yttrium aluminium garnet (Ce3+:YAG).

White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium-based red and blue emitting phosphors plus green emitting copper and aluminium doped zinc sulfide (ZnS:Cu, Al). This is a method analogous to the way fluorescent lamps work. This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger and more energy is therefore converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both approaches offer comparable brightness. Another concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin. 

Source:
TECH-FAQ.COM
WIKI.COM